Transport of electromagnetic waves in random media is known to be of great importance on both fundamental and applied levels. It may be described as a multiple scattering process, where the direction and phase of propagating waves is randomized due to spatial fluctuations of the refractive index. For strong scattering, interferences between scattered waves are significant, leading to weak and possibly strong localization. Strong localization is also called Anderson localization. Anderson localization is a general wave phenomenon that applies to the transport of electromagnetic waves, acoustic waves, quantum waves, spin waves, etc and it finds its origin in the wave interference between multiple-scattering paths.
Reports of Anderson localization of light in 3-dimensional (3D) random media have been made, such as in Wiersma et al., Nature, 390, pages 671-673 (1997) and in Störzer et al., Phys. Rev. Lett. 96, 63904 (2006), even though absorption may complicate interpretation of experimental results.
US patent application n. 2003/0133681 provides a waveguiding device and a method for guiding electromagnetic (EM) waves, in particular surface plasmon polaritons (SPPs), using strongly scattering random media exhibiting light localization. Also, the invention provides a cavity for providing resonance conditions for EM waves, in particular surface plasmon polaritons using strongly scattering random media exhibiting light localization. In a strongly scattering random medium with a high enough density of scatterers (so that the average distance between scatterers is smaller than the wavelength), EM waves can only exist in localized modes and can therefore not propagate. By forming regions free from scatterers in the regions with randomly distributed scatterers, the localization effects in scattering media can be utilized to guide propagating modes in these regions. The invention can be used to form compact integrated optical components and circuits.
Recently, localization of electromagnetic waves has been shown also in disordered 2D lattice.
In “Transport and Anderson localization in disordered two-dimensional photonic lattices”, written by Schwartz et al. and published in “Nature”, Vol. 446, pages 52-55 (2007), an experimental observation of Anderson localization in perturbed periodic potential is reported: the transverse localization of light is caused by random fluctuations on a two dimensional photonic lattice. In the article, it is demonstrated how ballistic transport becomes diffusive in the presence of disorder, and that crossover to Anderson localization occurs at a higher level of disorder.
U.S. Pat. No. 7,615,735 discloses a method and apparatus for random number generation using a scattering waveguide. The apparatus includes a light source for providing coherent light and a scattering waveguide for receiving the coherent light and providing scattered light. The relative position of the light source and the scattering waveguide are variable. The apparatus also includes a detector for forming at least one random number based on the scattered light.
Various techniques intended to increase the absorption efficiency of thin films exist (a film is considered “thin” when the ratio between the physical thickness of the film and the absorption mean free path—i.e. the characteristic length over which the amplitude of the electromagnetic wave is decreased by a factor of 1/e—is equal or lower than 1).
Known examples are:                thin film in which a randomly textured surface is present (improved coupling);        thin film in which metallic nanoparticles exhibiting surface plasmon resonances on the surface are deposited (improved coupling and near-field enhancement);        thin film in which there are periodic electromagnetic structures (improved coupling to guided modes and/or increased light-matter interaction by slow modes);        thin film in which there are graded-index structures (improved coupling).        
Most of these techniques have been developed for solar cell technologies to enhance light absorption. These approaches, however, hold for electromagnetic waves in general, owing to the scalability of Maxwell's equations (there is no intrinsic length scale), as well as for weakly scattering materials.
The following problems are commonly encountered in some of the above listed examples:                Small bandwidth: enhancement of the absorption is only observed on a small range of wavelengths due to the fact that it relies on single resonances of the structure or scatterer that are narrow in frequency (e.g. periodic electromagnetic structures, plasmon-enhanced absorption);        Poor coupling at large angles: the amount of light coupled to the structure at large angles is reduced or suppressed due to the high value of the reflection coefficients;        Poor suitability for ultra-thin films: the improvement of coupling efficiency requires a minimal thickness to be effective (randomly textured surface, graded-index structures);        Undesired absorption by additional material: the inclusion of defects (e.g. scatterers) that are absorbing electromagnetic waves reduces the amount of waves effectively in interaction with the film material (metallic nanoparticles for plasmon-enhanced absorption);        Poor suitability to large-scale low-cost production: the technique is extremely sensitive to any deviations from its original design and/or time and cost consuming for reproduction on large scales (periodic electromagnetic structures).        
In “Optical Absorption Enhancement in Silicon Nanohole Arrays for Solar Photovoltaics”, written by Sang Eon Han et al., published in Nano Letters, 2010, 10, pages 1012-1015, silicon periodic nanohole arrays as light absorbing structures for solar photovoltaics via simulation is investigated. To obtain the same ultimate efficiency as a standard 300 μm crystalline silicon wafer, it is found that nanohole arrays require twelve times less silicon by mass. Moreover, calculations show that nanohole arrays have an efficiency superior to nanorod arrays for practical thicknesses. With well-established fabrication techniques, nanohole arrays have great potential for efficient solar photovoltaics.
U.S. Pat. No. 4,554,727 is relative to a method for producing an optical enhanced thin film photovoltaic device. The method includes the steps of producing an active layer of semiconductor material wherein the surface of at least one side of the active layer is textured such that the surface includes randomly spaced, densely packed microstructures of predetermined dimensions of the order of the wavelength of visible light in the semiconductor material and attaching a reflecting surface directly to one side of the semiconductor material and making an ohmic contact to the material.
In “Engineering the randomness for enhanced absorption in solar cells” written by Stephan Far et al. and published in Applied Physics Letters 92, 171114 (2008), photon management by means of random textured surfaces is known to be a promising route to increase the light absorption in a solar cell. To date this randomness was only a posteriori assessed and related to the absorption. Here, the authors outline a meaningful strategy for a priori and purposely tailoring the randomness. By defining appropriate angular scattering functions and optimizing the surface profiles, it is shown that the number of absorbed photons can be enhanced by 55% compared to flat-surface solar cells.